How do I beat Bunker ?

Accepted Answer

I hung out near the top, right behind where he starts out, it gives pretty decent cover except against the XP loaders he chucks out once in a while. Maya can also use her phaselock ability to directly damage it since it can't be held, it doesn't do much but every bit helps because bunker has a LOT of health. When it hovers to the side you can take out the turrets it spawns to do a lot of damage to it.

Rocket launchers are good for dealing with it just for the sheer amount of health it has.

Other Answers

Another good spot to camp out at is right at the beginning of the map, by the vending machines. You need to wait for the Bunker to move around a bit to be able to get a shot, but you're almost completely protected from it's attacks & the turrets/loaders up top won't bother you.